The present invention relates to level measurement in industrial processes. More specifically, the present invention relates to measurement of product level height in a storage tank of the type used in industrial applications using a microwave level gauge.
Instrumentation for the measurement of product level (either liquids or solids) in storage vessels is evolving from contact measurement techniques, such as tape and float, to non-contact techniques. One technology that holds considerable promise is based on the use of microwaves. The basic premise involves transmitting microwaves towards the product surface and receiving reflected microwave energy from the surface. The reflected microwaves are analyzed to determine the distance that they have traveled. Knowledge of the distance traveled and storage vessel height allows determination of product level. Since it is known that microwaves travel at the speed of light, the distance that a microwave travels can be determined if the time of travel is known. The time of travel can be determined by measuring the phase of the return wave and knowing the frequency of the microwave that was transmitted. Further, the time of travel can be measured using well-known digital sampling techniques.
One standard in the process control industry is the use of 4-20 mA process control loops. Under this standard, a 4 mA signal represents a zero reading and a 20 mA signal represents a full scale reading. Further, if a transmitter in the field has sufficiently low power requirements, it is possible to power the transmitter using current from the two-wire loop. However, microwave level transmitters in the process control industry have always required a separate power source. The level transmitters were large and their operation required more power than could be delivered using the 4-20 mA industry standard. Thus, typical prior art microwave level transmitters required additional wiring into the field to provide power to the unit. This additional wiring was not only expensive but also was a source of potential failure.
A level transmitter measures height of product in a tank such as those used in industrial process applications. The level transmitter is coupled to a two-wire process control loop which is used for both transmitting level information provided by the level transmitter and for providing power to the level transmitter. The level transmitter includes a microwave antenna directed into the tank. A low power microwave source sends a microwave signal through the antenna into the tank. A low power microwave receiver receives a reflected microwave signal. Measurement circuitry coupled to the low power microwave source and to the low power microwave receiver initiates transmitting of the microwave signal and determines product height based upon the reflected signal received by the receiver. Output circuitry coupled to the two-wire process control loop transmits information related to product height over the loop. Power supply circuitry coupled to the two-wire process control loop receives power from the loop to power the level transmitter.
In one embodiment, the measurement circuitry includes a first clock coupled to the source for periodically initiating the microwave signal at a first clock rate. A second clock coupled to the receiver periodically gates the received signal at a second clock rate.